(1) Field of the Invention
The present invention relates to a methacrylic resin cast plate having a good and durable antistatic property and a process for the preparation thereof.
(2) Description of the Related Art
Methacrylic resin cast plates have a high transparency, good surface gloss, beautiful appearance, and good mechanical properties, and because of these excellent characteristics, methacrylic resin cast plates are widely used for illuminators, signboards, construction materials, nameplates, and decorative articles. However, these methacrylic resin cast plates have problems in that (a) since the surface resistance is high, the plates are very easily electrostatically charged, and dust, dirt and the like are readily attracted to the surface to degrade the appearance characteristics; (b) the plates are easily cracked; and, (c) the plates pass electricity to a human body or an electronic part.
As a means for imparting an antistatic property to a methacrylic resin cast plate, there can be mentioned (1) an internal addition of a surface active agent; (2) a surface coating with a surface active agent; (3) a surface coating with a silicon compound; and, (4) a surface modification by a plasma treatment. Among these methods, methods (3) and (4) are expensive, and thus methods (1) and (2) are generally adopted.
In methods (1) and (2), anionic, cationic and non-ionic surface active agents are used for attaining the above purpose, and it is known that a cationic compound containing a quaternary ammonium salt gives an excellent antistatic property. When a surface active agent as described above is coated on the surface of a methacrylic resin cast plate, to prevent damage to the surface condition of the plate, a delicate operation is necessary, and the so-treated cast plate has a problem in that the antistatic agent applied to the surface is readily separated by water washing or friction, resulting in a loss of the antistatic property.
In method (1) of the internal addition of a surface active agent, since the antistatic agent is incorporated or dispersed in the resin-forming starting material before the polymerization, the preparation steps can be simplified. However, in order to obtain a desired antistatic property, it is generally necessary to increase the amount of the antistatic agent added, and if the amount of the antistatic agent added is increased, the inherent transparency or mechanical characteristics of the methacrylic resin are degraded and the obtained antistatic property is readily lost by water washing or friction.
Use of a polymerizable monomer as the antistatic agent has been proposed and attempted, to overcome the above-mentioned defects of the internal addition method (see Japanese Examined Patent Publication No. 48-15,471 and No. 54-23,395). However, in this proposal, the attained antistatic property is not satisfactory, and since the adhesion strength between a plate obtained by the cast polymerization and a casting mold is increased, stable production becomes difficult.
As is apparent from the foregoing description, a methacrylic resin cast plate which can be prepared without a special preparation step, has a good and durable antistatic property and retains the inherent physical properties of a methacrylic resin has not been proposed.